Heat an 8-inch square waffle iron and preheat the oven to 200°. In a large bowl, whisk the ricotta with the milk, lemon juice,egg yolks and sugar. In a medium bowl, whisk the flour with the cornmeal, baking powder and salt. Whisk the dry ingredients into the ricotta mixture until combined. Stir in the melted butter.
In a large bowl, using an electric mixer, beat the egg whites until firm peaks form. Fold the egg whites into the waffle batter and add in the lemon zest and blueberries- stir just until incorporated.Coat the waffle iron with vegetable-oil spray and spoon about 1½ cups of the batter onto the waffle iron. Close and cook until the waffles are golden and crisp. Serve the waffles immediately or transfer them to a rack in the oven to keep warm.